This invention relates to energy absorbing bumpers, in particular such bumpers which are made out of elastic material and deform on impact to absorb the impact energy. Such bumpers are particularly useful for motor vehicles to prevent vehicle damage in the event of a collision.
A prior art energy absorbing bumper is described by Barton et al. in U.S. Pat. No. 3,638,985. The bumper described by Barton includes a support member, mounted to the vehicle, and a resilient member, which has a U-shaped cross section and is formed out of elastic material, such as rubber or dense urethane foam. The resilient structure is mounted to the support structure and has side walls which are thickest in the region of the support structure. The base portion of the resilient member is provided with a web, which in the event of unusually severe impact comes into contact with the support member and provides additional energy absorption when the ribs of the web structure are crushed against the support member. Deformation of the side walls of this prior art bumper absorbs impact energy from moderate force collisions, while deformation of the web provides energy absorption on the occurrence of a severe collision.
A major deficiency of the prior art impact absorbing bumper is that the absorption forces exerted by the bumper on the vehicle, and consequently the acceleration forces applied to the vehicle occupants varies in accordance with the ambient temperature. This variation in forces is a result of the variation in the elasticity of the material from which the resilient portion of the bumper is formed. FIG. 1 illustrates this variation of elasticity with temperature. The ratio of decelerating force F to vehicle mass G is plotted against temperature as curve a for a reaction cast polyurethane bumper and curve b for an ethylene-propylene-diene monomer (EPDM). From the curve of FIG. 1 it may be seen that at temperatures below approximately room temperature, there is a considerable increase in the deformation force exerted by the resilient bumper upon impact. When a moderate impact collision occurs with the prior art bumper at room temperature or above, the bumper has sufficient pliability to almost completely deform, therefore applying deceleration forces steadily during a large deformation and consequently applying a moderate deceleration force to the vehicle occupants. When the same force collision occurs at lower temperatures, the increased stiffness of the bumper material necessitates an increased force for total deformation. As a result, deformation of the bumper is not complete and the impact energy is dissipated during a lesser amount of bumper deformation. Consequently the deceleration forces exerted on the vehicle occupants are substantially increased.
FIG. 2 is a graph of bumper deceleration force plotted as a function of bumper deformation displacement s. The deformation forces associated with the prior art bumpers are plotted for four specific temperatures and indicated -10.degree. C, -5.degree., RT (room temperature), and +60.degree. C. As the graph illustrates in the event of a collision at lower temperature, for example -10.degree. C, the deformation of the bumper is approximately 40% of the deformation experienced at +60.degree. C. As a consequence, in order to dissipate the impact energy, it is necessary for the deceleration forces to be greatly increased. As can be seen from the graph, the deformation forces have a maximum at -10.degree. C which is approximately twice the maximum deformation force at room temperature or higher.
While the prior art bumper disclosed by Barton has two zones of deformation, the zones of deformation do not provide a compensation for the variation in deceleration force. The first zone of deformation disclosed by Barton is primarily effective to absorb energy in the event of a moderate impact. The second zone of deformation, which comprises compression of the web, is effective only in the event of a severe impact. In addition the second zone of deformation is rather small in actual displacement during the deformation process, necessitating extremely high forces to decelerate the vehicle in a short distance. This prior art design does not therefore compensate for variations in deformation force experienced as a result of temperature changes. The variation in deformation force for a moderate impact in the bumper described by Barton is similar to the variation to be expected in prior art bumpers wherein there is only a single cushion zone.
It is therefore an object of the present invention to provide a new and improved energy absorbing bumper.
It is a further object of the present invention to provide such a bumper wherein the impact absorbing forces exerted by the bumper are relatively constant over a wide range of temperatures.